Low-loss acoustic wave filter device

Abstract

An acoustic wave filter device with low loss and low passband ripple characteristics. The filter comprises three bidirectional electroacoustical transducers disposed along a common axis on the surface of a substrate of piezoelectric material. The two outer transducers of the three function as input transducers and the center transducer functions as an output transducer. One of the input transducers is of the antisymmetric type and has a transfer characteristic that differs in phase angle by ninety degrees with respect to the other input transducer, which is of the symmetric type. This ninety-degree difference applies at all frequencies, so that any energy reflected from the output transducer and again reflected by an input transducer will be subject to a phase shift of 180 degrees in the case of energy reflected from the antisymmetric input transducer, and no phase shift in the case of the symmetric input transducer. Accordingly, these triple-transit echoes will cancel at the output transducer, for all frequencies. For the primary energy propagation from the input transducers to the output transducer, a ninety-degree phase shifter in one input circuit ensures that the signals from the two input transducers are in phase when they reach the output transducer.

We claim: 
     
       1. An acoustic wave filter comprising: a substrate of piezoelectric material;   a symmetric electroacoustic output transducer disposed on said substrate;   a pair of electroacoustic input transducers disposed on said substrate at equal distances on opposite sides of said output transducer, one of said input transducers being symmetric and the other being antisymmetric, the two having transfer functions that differ by a ninety-degree angle and having an aperture substantially equal to that of the output transducer; and   ninety-degree phase shifting means coupled to one of said input transducers, to compensate for the ninety-degree phase shift introduced by said antisymmetric input transducer;   whereby signals reflected from said output transducer to said antisymmetric input transducer and back again will return with a 180-degree phase shift and will thereby cancel signals reflected from said output transducer to said symmetric input transducer and back again, and whereby the cancellation effect is relatively independent of frequency.   
     
     
       2. An acoustic wave filter as set forth in claim 1, and further including: two U-shaped multistrip couplers each positioned to surround one of said input transducers, to further reduce losses by causing said input transducers to function as unidirectional devices.   
     
     
       3. An acoustic wave filter as set forth in claim 1, wherein: said transducers are amplitude-weighted to provide a desired frequency response characteristic.   
     
     
       4. An acoustic wave filter, comprising: a substrate of piezoelectric material;   an electroacoustic interdigital transducer disposed on said substrate and serving as an output transducer;   a pair of electroacoustic interdigital transducers serving as an output transducer;   a pair of electroacoustic interdigital transducers serving as input transducers and disposed on said substrate at equal distances on opposite sides of said output transducer, with the transducer fingers of said three transducers parallel to each other and with said three transducers aligned on a common axis and having substantially the same aperture; and   ninety-degree phase shifting means coupled to one of said input transducers;   wherein one of said input transducers is symmetric and the other is antisymmetric, the two having transfer functions that differ by a ninety-degree phase angle, and wherein said ninety-degree phase shifting means compensates for the phase shift introduced by said antisymmetric transducer and ensures that signals from both of said input transducers reach said output transducer with the same phase angle;   and whereby signals reflected from said output transducer to said antisymmetric input transducer and back again will return with a 180-degree phase shift and will thereby cancel signals reflected from said output transducer to said symmetric input transducer and back again, and the cancellation effect is relatively independent of frequency.   
     
     
       5. An acoustic wave filter as set forth in claim 4, and further including: two U-shaped multistrip couplers each positioned to surround one of said input transducers, to further reduce losses by causing said input transducers to function as unidirectional devices.   
     
     
       6. An acoustic wave filter as set forth in claim 4, wherein: said transducers are amplitude-weighted to provide a desired frequency response characteristic.   
     
     
       7. An acoustic wave filter comprising: a substrate of piezoelectric material;   a symmetric electroacoustic input transducer disposed on said substrate;   a pair of electroacoustic output transducers disposed on said substrate at equal distances on opposite sides of said input transducer, one of said output transducers being symmetric and the other being antisymmetric, the two having transfer functions that differ by a ninety-degree phase angle and having an aperture substantially equal to that of said input transducer; and   ninety-degree phase shifting means coupled to one of said output transducers, to compensate for the ninety-degree phase shift introduced by said antisymmetric output transducer;   whereby signals reflected from said antisymmetric output transducer to said input transducer and back again be subject to a 180-degree phase shift and will thereby cancel signals reflected from said symmetric output transducer to said input transducer and back again.   
     
     
       8. An acoustic wave filter as set forth in claim 7, and further including: two U-shaped multistrip couplers each positioned to surround one of said output transducers, to further reduce losses by causing said output transducers to function as unidirectional devices.   
     
     
       9. An acoustic wave filter as set forth in claim 7, wherein: said transducers are amplitude-weighted to provide a desired frequency response characteristic.